Human Pigmentation Flashcards
Plasticity/Adaptability
The ability of an individual organism to short-term make physiological changes in response to exposure to stressful environmental conditions
- often associated with changes that come to be over the period of growth and development
Adaptation
The ability of an individual organism to make anatomical or physiological changes in response to exposure to stressful environmental conditions
- often associated with changes that come to be by altering patterns of growth and development
Acclimatization
State reverts after environmental stress is removed
What is the largest organ of the body?
Skin
- our most conspicuous polymorphism
The many functions of Skin:
- protection
- thermoregulation
- Vitamin D synthesis
What is the pigment primarily responsible for light-dark coloration of most animals?
Melanin
All people have the same number of Melanocytes, but what are melanocytes?
Cells that make melanin
Skin color depends on how much and which type of _____ is produced
Melanin
Eumelanin
molecule responsible for most variation in human skin pigmentation
- Dark: Brown to black
- Concentration accounts for skin’s darkness
Pheomelanin
- Lighter: yellow to reddish brown
- Accounts for skin’s reddish-brown color
Lightly pigmented people are while (the color of connective tissue)
A pink hue comes from blood hemoglobin showing through
Our skin has a layer structure. Melanocytes are near the base of the epidermis
- Melanocytes are embedded between the basal layer and the overlying stratum spinosum
- As melanosomes fill up with melanin, the pigment gets distributed to new skin cells through the dendrites
Skin pigmentation is determined by:
- rate of production
- distribution
- size of melanin packets
There are a lot of biological factors involved in melanin and skin pigmentation
- some are genetic
- some are hormonal
Are all babies born with lighter skin than they will have as adults?
Yes; because melanocytes have to “ramp up” into production
Human biologists measure skin color using spectrophotometers or similar equipment
This measures how much light is reflected at a specific wavelength
- light colors reflect more light
- darker colors absorb more light
Human skin pigmentation is an excellent example of a continuous and complex phenotype
- skin pigmentation is highly polymorphic
- continuous: the polymorphisms have gradient-style variation (there are an infinite number of intermediate states)
- complex phenotypes are influenced by environment (genetic-adaptation=constitutive pigmentation; acquired-acclimatization=facultative pigmentation)
- skin pigmentation is polygenic (many genes)
Is skin pigmentation highly heritable?
Yes
- BUT the genetic control over the phenotype is not straightforward
The TYR gene makes the enzyme tyrosinase, which is involved in _____?
melanin production at a few different stages
Albinism
caused by the lack of melanin
- over 100 distinct genetic causes/mutation in TYR
4-6 genes identified for skin pigmentation genetics?
1.) TYR: Tyrosinase
2.) MC1R: Melanin production
3.) OCA2: P protein, has to do with melanosome function
4.) OCA4
5.) SLC24A5
Same reflectance value from different genotypes
- some alleles can be MASKED
Some genes involved in skin pigmentation are also involved in eye and hair color
Melanin is also involved in eye color
- eye darkness vs. lightness
OCA2 (Chromosome 15) was identified by earliest studies to be the dominating factor for eye color determination
- now known that other genes are involved (HERC2) that don’t affect melanin directly
MCR1 contains regions where the allele present is associated with whether or not a person has green eyes
TYR can stop melanin production at the highest hierarchical level
UVR
Energy with wavelength between visible light and x-rays
- has potential to alter biological processes
UVR can be sub-divided into:
UVA: longest wavelength, potential to penetrate skin deeply
UVB: intermediate wavelength, potential to penetrate skin less deeply
UVC: shortest wavelength, atmosphere absorbs/scatters
More and larger melanosomes permit___
less UVR penetration
Darkest human pigmentation can block ca. 95% UVR
Light pigmentation blocks less than 50% UVR
UVR negative effects
- UVR can damage the skin
- sunburn (caused especially by UVB)
- destroys skin elasticity (UVA)
- can damage molecules and compounds inside the body
- all UV wavelengths can damage DNA
- This is one cause of melanoma
Short-term, reversible response to UVR penetration
Tanning
Tanning
When UVR exposure exceeds a threshold, your body starts a self-regulation mechanism
- existing melanin skin darks (UVA)
- melanin production is increased (UVB) (at this point, you already have DNA damage)
All people can tan. All people can burn.
TRUE; the amount you see the different will depend on how dark someone is
Folate (folic acid)
B vitamin required to produce nucleic acids
- an process involving cell proliferation
Linked to many birth defects
- plays an important role n brain and spinal cord
Required for spermatogenesis
Myelin and serotonin
- brain function
Positive effects of UVR
Catalyze production of compounds inside the body
Vitamin D synthesis
- UVB exposure initiates pre-vitamin D production
UVR intensity varies with latitude and altitude
TRUE
Earth is a sphere
Cross-sectional area of “a ray of sunlight” increases with distance from equator
Earth has a thick atmosphere that absorbs and scatters UVR
- a ray of sunlight hitting surface at equator passes through less atmosphere than one hitting surface at high latitude
- altitude alos affects UV intensity for this reason
Intensity of UV radiation increases near equator, and creates at high altitude localities near equator
Human populations living in some parts of the world are at risk of UVR overexposure while…
others are at risk of underexposure
At high latitudes
Season affects UVR
- each currently has an axial tilt of about 23 degrees
- Axial tilt accounts for variation in length of daylight (near the equator, there is no significant variation in day length throughout)
Overexposure risk in the summer and underexposure risk in the winter example
True
There is a very strong correlation between pigmentation and UVR strength… how? Why?
Pigmentation in the skin serves as a natural defense mechanism against the harmful effects of UV radiation. The darker the skin, the more melanin present, providing greater protection against sun damage
Non-human apes are lightly pigmented under their hair (they have little melanin in their skin)
- they can develop facultative pigmentation on their more exposed areas
- or exposed areas are permanently pigmented
The evolution of darkly pigmented skin is probably linked to hairlessness
- responses to increased temperature regimes with expansion into shadeless environments
- hair prevents effective evaporative cooling
When did hominins get naked?
When they moved into shadeless environments
- sometime near the beginning of the Pleistocene
Homo erectus
- shows adaptation to hot environments in body proportions
- associated with dry and open grassland fauna
- researchers propose that the species was dark and naked
How would darkly pigmented skin have contributed to differential fitness?
Proposed hypotheses:
- sunburn
- folate protection
- skin cancer
The folate hypothesis
Argues that protection of folate stores enhanced fitness of early hominins with darkly pigmented skin
- folate depletion in females is associated with the development of neural tube defects in the fetus
- darkly pigmented women have lower incidence of neural tube related birth defects
- there is an annual peak of babies born with neural tube defects that were conceived in the summer months
- in men, folate deficiency is associated with impaired spermatogenesis
How would lightly pigmented skin have contributed to differential fitness?
- Darwin thought it was sexual selection
- increased vitamin D synthesis
Support for the Vitamin D hypothesis
Vitamin D and bone formation
- Calcium absorption
- osteoblast and chondrocyte activity
- can lead to rickets
Vitamin D vital to immune system function
Independent evolution of lightly pigmented skin
- molecular signature of purifying selection
